In an inkjet recording apparatus that may be applied to an imaging apparatus such as a printer, a facsimile machine, a copier, and a plotter, for example, a small capacity sub tank is implemented on a carriage, a large capacity main cartridge (main tank) is implemented in a main body of the apparatus, and an apparatus for supplying ink from the main cartridge at the apparatus main body to the sub tank is provided.
In Japanese Patent Laid-Open Publication No. 2003-53993, a sub tank including a movable part that is made of a deformable film sheet, a spring that is adapted to supply negative pressure, and a supply/exhaust path that supplies ink and discharges mixed gas is provided. In this sub tank, the supply/exhaust path is positioned so that interference with the movable part and the spring may be avoided.
In Japanese Patent Laid-Open Publication No. 2002-86748, another exemplary sub tank is disclosed, this sub tank including an ink chamber that deforms according to ink volume while maintaining a negative pressure of the ink, an ink entering unit and exhaust unit that are implemented at an upper portion of the ink chamber, and an ink supply unit that is implemented at a lower portion of the ink chamber. The ink entering unit includes a valve seat made of an elastic material and having an ink entering path, a supply valve having a valve portion, and an elastic member that seals together the valve portion and the valve seat with pressure to block the ink entering path. The exhaust unit includes a seal portion made of elastic material and having a closed slit at its center.
In Japanese Patent Laid-Open Publication No. 2003-1846, a sub tank and a liquid supply apparatus including such a sub tank are disclosed, the sub tank including a negative pressure generation unit that expands and contracts by the supplying and discharging of a fluid therein, an atmospheric release unit that opens the sub tank to the atmosphere, and an ink supply unit for supplying ink. In this prior art example, upon supplying liquid from a main tank to the sub tank, the interior portion of the sub tank is exposed to the atmosphere by means of the atmospheric release unit, the fluid is supplied to the negative pressure generation unit so that it expands, and the liquid is thereby supplied to the sub tank. After the liquid is supplied to the sub tank, the atmospheric release unit is closed, and the negative pressure unit is contracted so that a negative pressure is generated within the sub tank.
In an imaging apparatus implementing a sub tank of the conventional art, a supply tube that supplies ink from the main tank to the sub tank, and a flexible film member that is used as a damper for controlling the pressure fluctuation within the sub tank are implemented. With long term use, air gradually penetrates through such components, and the air may accumulate in the sub tank. Also, a small amount of air may enter the main tank upon its detachment, and this air may also be supplied to the sub tank along with ink.
Accordingly, in the sub tank of Japanese Patent Laid-Open Publication No. 2003-53993, the ink supply path is also used as an air exhaust path so that the air in the sub tank may be discharged. However, in this case, when the imaging apparatus is not used for a long period of time, ink adhering to the entrance portion of the supply exhaust path may grow viscous and the path may be sealed by this ink.
Thus, it is preferred that the ink entering path and the exhaust unit for discharging air within the sub tank be separately implemented in the sub tank as in the case of Japanese Patent Laid-Open Publication No. 2002-86748. However, even in this prior art example, when ink enters the exhaust unit, the same effect as that described above may occur when the imaging apparatus is not used for a long period of time; that is, the ink at the entrance portion of the exhaust unit may grow viscous and may seal the exhaust path.
A sub tank is preferably arranged to have an ink accommodating portion for accommodating ink, and an air flow path for discharging air from the ink accommodating portion, in which sub tank the entrance portion of the air flow path is positioned above the liquid level of ink accommodated in the ink accommodating portion so that ink does not enter the air flow path.
However, when the imaging apparatus is in use, ink may enter the air flow path due to movement of the carriage which causes the liquid level of ink in the sub tank to fluctuate. As a result, ink may adhere to a sealing member of an air releasing valve that is used for opening and closing the air flow path, and the ink may grow viscous so that sealing may not be realized and the air flow path may be blocked.
Also, in a case where a deformable film sheet is used to seal a path in the sub tank as in the case of Japanese Patent Laid-Open Publication No. 2003-53993, the air flow path may be formed into a trench that is sealed by the film sheet. In such configuration, the ink may be pulled into the air flow path due to the capillary effect.
Additionally, in the sub tank disclosed in Japanese Patent Laid-Open Publication No. 2003-1846, since the negative pressure generation unit that expands and contracts by the supplying and discharging of fluid is implemented in the sub tank, a mechanism for supplying the fluid for expanding and contracting the negative pressure generation unit is needed aside from the mechanism for supplying the liquid to the sub tank in order for the negative pressure to be generated within the sub tank. Thereby, the structure of the sub tank may be quite complicated.
Similarly, in the sub tank of Japanese Patent Laid-Open Publication No. 2002-86748, since a flexible container that accommodates ink is implemented inside the case, and the flexible container is expanded and contracted by suction/atmospheric release of the case, the structure of the negative pressure generation unit may be complicated.
Also, in an inkjet recording apparatus, ink may adhere in the recording head nozzles due to the increase in the ink viscosity and the drying of ink. Accordingly, in Japanese Patent Laid-Open Publication No. 2002-234189, and in Japanese Patent Publication No. 8-2651, disclosures are made pertaining to a restoration operation for restoring the states of recording head nozzles by capping the nozzles with a cap at predetermined timings and absorbing ink from the nozzles.
Also, in Japanese Patent Laid-Open Publications No. 5-270004, No. 8-156282, and No. 2001-71451, disclosures are made of inkjet recording apparatuses that have ink supplied thereto directly from an ink cartridge (main tank) without using a sub tank, in which an ink end (out of ink condition), including a near end, for ink in a cartridge can be detected, and when one of plural inks in different colors is detected to have reached its end, the printing mode is switched from full color printing to monocolor printing.
In a case where a restoration operation is performed in a liquid discharge apparatus including a conventional sub tank as described above, bubbles are likely to be generated when air is present in the sub tank so that the nozzles may easily come off. Also, it is noted that control of the negative pressure may be difficult, and ink discharge characteristics may be susceptible to fluctuations.
In an imaging apparatus using a liquid storage tank (main tank) and a sub tank, as the ink in the sub tank is consumed through ink discharge by the ink discharge heads, and through the restoration operation, the sub tank has to be appropriately replenished with ink from the main tank. Further, when the sub tank implements a flexible film member and an elastic member to generate a negative pressure as is described above, the capacity of the sub tank changes, and thereby, the amount of ink remaining in the sub tank may not be accurately detected.
Also, according to tests conducted by the inventors of the present invention, it has been determined that when the capacity of the sub tank is significantly reduced, a hysteresis in the capacity change occurs at the time the elastic member for generating a negative pressure is contracted and at the time the elastic member returns to its initial state. When such hysteresis occurs, instability is created in the control of the negative pressure, and the liquid discharge characteristics become unstable. In turn, deviations in the ink ejection and differences in the ink discharge speed may occur.